In the case of a large number of field devices of the described kind, the measuring transducer is so activated during operation, for production of the measurement signal, additionally by an activating signal generated at least at times by the operating and evaluating circuit that it acts on the medium in a manner suited for the measurement, at least directly, or, however, practically directly via a probe directly contacting the medium, in order to evoke there reactions appropriately corresponding to the measured variable to be registered. The activating signal can, in such case, be appropriately controlled, for example with respect to an electrical current strength, a voltage level and/or a frequency. Examples of such active measuring transducers, thus measuring transducers appropriately converting an electrical activating signal in the medium include, especially, flow measuring transducers serving for measuring media flowing at least at times, and having e.g. at least one coil producing a magnetic field activated by the activating signal or at least one ultrasonic transmitter activated by the activating signal, or fill and/or limit level transducers serving for measuring and/or monitoring fill level in a container, such as e.g. those with a freely radiating, microwave antenna, Goubau-line or vibrating immersion element.
For accommodating the measuring device electronics, field devices of the described kind further include a comparatively robust, especially impact-, pressure- and/or weather-resistant, electronics housing. This can, as proposed e.g. in U.S. Pat. No. 6,397,683 or WO-A 00/36379, be arranged remotely from the field device and be connected therewith only via a flexible cable; it can, however, also be, as disclosed in EP-A 903 651 or EP-A 1 008 836, one arranged directly on the measuring transducer or on a measuring transducer housing separately housing the measuring transducer. Often then the electronics housing serves, as shown, for example, in EP-A 984 248, U.S. Pat. Nos. 4,594,584, 4,716,770 or 6,352,000, also for accommodating, as well, various mechanical components of the measuring transducer, components such as e.g. membrane-, rod- or tube-shaped, deformation- or vibration-elements operationally deforming as a result of mechanical effects; compare, in this connection, also the initially mentioned U.S. Pat. No. 6,352,000.
In the case of measuring devices of the aforementioned kind, the respective measuring device electronics are usually electrically connected via corresponding electrical lines to a superordinated, electronic, data processing system, which is, most often, arranged spatially remotely and also spatially distributedly and to which the measured value produced by the measuring devices is transmitted, essentially in real-time, by means of a measured value signal appropriately bearing such measured value.
Measuring devices of the described kind are, additionally, usually connected together and/or with appropriate, electronic process control units by means of a data transmission network provided as a part of the superordinated data processing system. The data processing system can involve, for example, one or more programmable logic controllers installed on-site, or process control computers installed in a remote control room, to which the measured values produced by means of the measuring device, digitized in suitable manner and appropriately coded, are sent. By means of such process control computers, the transmitted measured values can be further processed and visualized as corresponding measured results e.g. on monitors and/or converted into control signals for other field devices embodied as control devices, e.g. magnetic valves, electric motors, etc. Since modern measuring arrangements can mostly also be monitored, and, as required, controlled and/or configured, by such control computers, operating data pertinent to the measurement transmitter feeding device and/or to the measuring device are equally transmitted in appropriate manner via the aforementioned data transmission networks, which are, most often, hybrid as regards transmission physics and/or transmission logic. Accordingly, the data processing system serves usually also for conditioning the measured value signal delivered from the measuring device. The conditioning is done according to the requirements of the subsequently placed data transmission networks and includes, for example, suitable digitizing and, as required, conversion into appropriate telegrams, and/or evaluation on site. For this propose, included in such data processing systems are evaluating circuits electrically coupled with the respective connecting lines. These evaluating circuits pre- or further-process, as well as, where necessary, suitably convert, the measured values received from the respective measuring device. Serving for the data transmission in such industrial data processing systems, at least sectionally, especially serially, are fieldbusses, such as e.g. FOUNDATION FIELDBUS, RACKBUS-RS 485, PROFIBUS, etc., or, for example, also networks based on the ETHERNET standard, as well as the corresponding, usually comprehensively standardized, transmission protocols.
Besides the evaluation circuits required for the processing and converting of the measured values delivered by the respectively connected measuring devices, such superordinated data transmission systems most often also include, for supply of the connected measuring devices with electrical energy, electrical supply circuits, which make available for the respective measuring device electronics an appropriate supply voltage, especially a supply voltage fed directly from the connected fieldbus, for driving electrical currents flowing through the electrical lines connected thereto as well as the respective measuring device electronics. A supply circuit can, in such case, be accommodated (for example, as assigned exactly to one measuring device and joined together with the evaluating circuit assigned to the particular measuring device—for example for a corresponding fieldbus adapter) in a common electronics housing, e.g. one embodied as a top-hat module. It is, however, also quite usual to place supply circuits and evaluating circuits in separate electronics housings, perhaps spatially removed from one another, and to appropriately connect them together via external lines.
Examples providing further information for such measuring devices known per se to those skilled in the art or for such measuring arrangements, such as are embodied in an interaction of measuring device and a corresponding data processing system are described extensively and in detail in, among others, WO-A 03/048874, WO-A 02/45045, WO-A 02/103327, WO-A 02/086426, WO-A 01/02816, WO-A 00/48157, WO-A 00/36379, WO-A 00/14485, WO-A 95/16897, WO-A 88/02853, WO-A 88/02476, U.S. Pat. Nos. 7,134,348, 7,133,727, 7,075,313, 7,073,396, 7,032,045, 6,854,055, 6,799,476, 6,776,053, 6,769,301, 6,577,989, 6,662,120, 6,640,308, 6,574,515, 6,535,161, 6,512,358, 6,487,507, 6,480,131, 6,476,522, 6,397,683, 6,352,000, 6,311,136, 6,285,094, 6,269,701, 6,236,322, 6,140,940, 6,014,100, 6,006,609, 5,959,372, 5,796,011, 5,742,225, 5,706,007, 5,687,100, 5,672,975, 5,604,685, 5,535,243, 5,469,748, 5,416,723, 5,363,341, 5,359,881, 5,231,884, 5,207,101, 5,131,279, 5,068,592, 5,065,152, 5,052,230, 4,926,340, 4,850,213, 4,768,384, 4,716,770, 4,656,353, 4,617,607, 4,594,584, 4,574,328, 4,524,610, 4,468,971, 4,317,116, 4,308,754, 3,878,725, US-A 2006/0179956, US-A 2006/0161359, US-A 2006/0112774, US-A 2006/0096390, US-A 2005/0139015, US-A 2004/0117675, EP-A 1 158 289, EP-A 1 147 463, EP-A 1 058 093, EP-A 984 248, EP-A 591 926, EP-A 525 920, DE-A 44 12 388 or DE-A 39 34 007.
In the case of modern measuring devices of the type being discussed, often involved are so-called two-conductor field devices, thus such measuring devices, in the case of which the measuring device electronics is electrically connected with the superordinated data processing system solely via a single pair of lines and in the case of which both the electrical energy fed from outside into the measuring device as well as also the measured values produced by the measuring device are transmitted via the single pair of electrical lines. Often, in the case of two-conductor field devices, the current driven by the external supply voltage and flowing during operation in the current loop formed by means of the pair of lines and correspondingly tuned on the measuring device side by means of load modulation, serves as the signal carrier bearing the measured values. In numerous industrial applications, it is established practice, in such case, to embody the one-pair connecting line as a so-called 4 mA to 20 mA current loop. In such case, the instantaneous measured value is, as is known, represented analogy by means of an instantaneously tuned, electrical current strength, which lies within a predetermined range between 4 mA and 20 mA (=milliamperes). The measuring device electronics includes, for such purpose, usually, in each case, a current controller, through which the loop current flows and which serves for setting or modulating, and, as required, also clocking, or switching, of the current, an internal operating and evaluating circuit for operating the measuring device, as well as an internal supply circuit lying at an internal input voltage of the measuring device electronics, divided from the supply voltage and feeding the internal operating and evaluating circuit and having at least one voltage controller, through which a variable component of the loop current flows. The voltage controller provides in the measuring device electronics an internal, useful-voltage controlled to be essentially constant at a predeterminable voltage level. Examples of such two-conductor field devices are disclosed in, among others, WO-A 03/048874, WO-A 02/45045, WO-A 02/103327, WO-A 00/48157, WO-A 00/26739, WO-A 94/20940, U.S. Pat. Nos. 6,799,476, 6,577,989, 6,662,120, 6,574,515, 6,535,161, 6,512,358, 6,480,131, 6,311,136, 6,285,094, 6,269,701, 6,140,940, 6,014,100, 5,959,372, 5,742,225, 5,672,975, 5,535,243, 5,416,723, 5,207,101, 5,068,592, 5,065,152, 4,926,340, 4,656,353, 4,317,116, US-A2006/0161359, US-A 2004/0117675, EP-A 1 147 841, EP-A 1 058 093, EP-A 591 926, EP-A 525 920, DE-A 44 12 388, or DE-A 39 34 007.
A special problem of such two-conductor field-devices is, however, that essentially only a single, physical, information carrier—here the electrical current, respectively its level, flowing in the line-pair—is provided for the measured values to be transmitted and that, as a result, instantaneously always only exactly a single measured value of the particular measuring device can be evaluated in the superordinated evaluating unit connected via the line-pair. In other words, in the case of such two-conductor field devices, multiple measured values—be they of the same and/or different type—are transmitted in all cases sequentially in real time. For the case, however, arising quite often, wherein the field device is a multivariable measuring device, for example a Coriolis mass flow/density meter, which is capable of ascertaining highly accurately and quasi in parallel, with high update rates, measured values of various kinds, for example the instantaneous and/or time-integrated mass flow, the instantaneous density and/or an instantaneous viscosity of the medium, the potential of this measuring device would not be able to be utilized in full measure, as a result being limited by the conventional, actually otherwise very advantageous, two-conductor technology.
A further problem of such measuring devices implemented in two-conductor technology is, additionally, that the electrical power actually usable by the measuring device electronics—for short, the “available power”—can fluctuate during operation over a wide range in practically unpredictable manner, as a result of changes of the measured variable as a function of time. As a result of this, only the power “available” at minimum nominal current, e.g. about 4 mA, can be safely depended on as the nominal power.
Making the situation more difficult is the fact that a large number of modern field devices of the described kind must, additionally, be so embodied electrically that they meet requirements for explosion safety. Especially, such field devices are operated, in the case of required intrinsic explosion safety, with such a low electrical “available” power, that, in order not to bring about ignition conditions, sparks or arcs cannot be issued electrically, with also the electrical energy stored in all possible cases directly in the measuring device having to be taken into consideration in the total energy and power balance. As a result of this, also the storage of momentarily excess electrical energy is only possible to a very limited, most often only insufficient, degree. Intrinsically safe explosion protection is given, for example according to the European standards EN 50 014 and EN 50 020, when electronic devices are so constructed that they satisfy the ignition protection type “Intrinsic Safety (Ex-i)” defined therein. In accordance with this ignition protection type, thus, electrical currents, voltages and powers arising in the field device must at all times not exceed specified current, voltage and power, limit values. These three limit values are so chosen that, in the case of malfunction, for example in the case of short circuiting in the measuring device electronics, the maximally liberated amount of energy is not sufficient to produce an ignition-capable spark. Usually, in the case of intrinsically safe field devices, the electrical power is not permitted to exceed 1 W (=watt) of electrical power. The voltage can be kept below the specified limit values e.g. by Z-diodes, the current e.g. by resistors and/or fuses, and the power by appropriate combinations of voltage and current limiting components and/or appropriate power controllers. In certain circumstances, it is also possible to provide in the individual measuring device electronics, as described e.g. in U.S. Pat. No. 7,113,375, corresponding safety circuits, which enable, additionally, an automatic shut-down at least of malfunctioning and/or malfunction-causing components, or assemblies, of the measuring device electronics.
In order to cope with the time-dependent lack of available power, modern two-conductor measuring devices, especially those with (4 mA to 20 mA) current loops, are consequently, in part, so designed, that their device functionality, implemented by means of a microcomputer provided in the evaluating and operating circuit, is variable, and, as a result, the operating and evaluating circuit, which mostly uses little power anyway, can be adapted to the momentarily available power. In certain cases, also individual circuit components of the particular measuring device electronics can be shifted during operation into a standby mode requiring less power or, as required, even be completely turned off for a time.
Further solutions for implementation of measurement arrangements wherein, on the one hand, the measuring device, which may be embodied with intrinsic safety, can be supplied permanently with electrical power and, on the other hand, can be connected to the two-conductor interface formed in the interaction of supply and evaluating circuits within the superordinated data processing system, are proposed, for example, in U.S. Pat. No. 6,684,340 or U.S. Pat. No. 6,472,884. Accordingly, the measuring device is fed with electrical energy by appropriate, external supply circuits, in each case, via two line-pairs formed, in each case, as two-conductor current loops, wherein the variable currents flowing, in each case, in the line-pairs, are controlled largely independently by the particular measuring device. Further, it is provided in the proposed measuring arrangements that the instantaneous current strength of one of the currents is set as a function of the measured value, while the instantaneous current strength of the other current is set essentially as a function of the instantaneous energy requirement of the measuring device.